Light-emitting diode with hyperbolic metamaterial

ABSTRACT

A light-emitting diode includes a first semiconductor region of one of p- or n-conductivity types, a second semiconductor region of the other one of p- or n-conductivity types, forming a p-n junction with the first semiconductor region, and a quantum well layer at the p-n junction between the first and second semiconductor regions. A hyperbolic metamaterial structure is provided in the second semiconductor region. The hyperbolic metamaterial structure is coupled to the quantum well layer for extracting light from the quantum well layer. The hyperbolic metamaterial structure may be patterned to provide an array of nanoantennas to apodize the emitted beam, and to control the polarization state of the emitted beam.

REFERENCE TO RELATED APPLICATION

The present invention is a continuation of U.S. patent application Ser.No. 16/384,759 filed on Apr. 15, 2019 and incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present disclosure relates to optical components and modules, and inparticular to light-emitting diodes and light-emitting diode arrays.

BACKGROUND

Light-emitting diodes (LEDs) are promising light sources for lighting,display applications, indicators on instrument panels, etc. A growinginterest in wearable devices and their numerous applications (augmentedreality, virtual reality displays, etc.) created a need forhigh-efficiency, miniature visual display devices. Miniature visualdisplay panels can be used in head-mounted displays (HMDs), near-eyedisplays (NEDs), and other wearable displays, and may enable reductionof size and weight of the HMDs and NEDs.

An efficiency of an LED is determined by a ratio of radiative andnon-radiative recombination of electrons and electron holes at a p-njunction of the LED. The non-radiative recombination rate typicallyincreases as the size of an LED pixel decreases. One reason for this isa comparatively high density of surface states. Smaller LED pixelspresent higher current densities, which may cause possible non-radiativerecombination of carriers and/or high leakage current of the LED. It isalso known that LED edges may act as a strong perturbation of theperiodicity of a crystal lattice. This induces electronic states withinthe semiconductor gap that become non-radiative recombination centers insmall LED chips. Due to these deleterious effects, a miniature LEDdisplay panel may suffer from a comparatively low conversion efficiencyand, consequently, low image brightness, high energy consumption,overheating, etc. Low image brightness and high energy consumption maylimit the use LED panels in HMDs, NEDs, head-up displays, and otherdisplay systems.

SUMMARY

In accordance with the present disclosure, there is provided alight-emitting diode (LED) comprising a first semiconductor region ofone of p- or n-conductivity types. A second semiconductor region of theother one of p- or n-conductivity types forms a p-n junction with thefirst semiconductor region. A quantum well layer is provided at the p-njunction between the first and second semiconductor regions. Ahyperbolic metamaterial structure in the second semiconductor region iscoupled to the quantum well layer.

The hyperbolic metamaterial structure may include a stack of alternatingmetal and semiconductor layers. The LED may have lateral dimensions ofno greater than 25×25 micrometers, for example.

The hyperbolic metamaterial structure may include an array of featurescoupled to the quantum well layer. The array of features may have aplasmonic resonance optical frequency within a spectral gain band of thequantum well layer. The array of features may have a spatially variantpitch and/or a spatially varying duty cycle. A between the quantum welllayer and features of the array may vary across the array.

In some embodiments, the array of features is two-dimensional. Eachfeature of the array may include at least one of a cylinder feature, across feature, or a chevron feature, for example. The array may includea plurality of sub-arrays of features, each sub-array comprising anarray of grating lines. The features of the array may be configured toprovide a pre-defined polarization of emitted light. Features of thearray may extend from the quantum well layer and into the secondsemiconductor region. At least some of the features of the array may beinclined towards a center of the array.

In accordance with the present disclosure, there is provided a displaydevice comprising an array of light-emitting diodes. Each light-emittingdiode may include a first semiconductor region of one of p- orn-conductivity types; a second semiconductor region of the other one ofp- or n-conductivity types, forming a p-n junction with the firstsemiconductor region; a quantum well layer at the p-n junction betweenthe first and second semiconductor regions; and a hyperbolicmetamaterial structure in the second semiconductor region. Thehyperbolic metamaterial structure is coupled to the quantum well layer.The display device may further include an element having optical poweroptically coupled to the array of light-emitting diodes and spaced aparttherefrom for redirecting optical beams emitted by the array oflight-emitting diodes.

In some embodiments, the hyperbolic metamaterial structure includes anarray of features coupled to the quantum well layer. At least some ofthe features of the array may be inclined towards center of the array.

In accordance with the present disclosure, there is further provided amethod of manufacturing a light-emitting diode (LED). The methodincludes providing a die comprising: a first semiconductor region of oneof p- or n-conductivity types; a second semiconductor region of theother one of p- or n-conductivity types forming a p-n junction with thefirst semiconductor region; and a quantum well layer at the p-n junctionbetween the first and second semiconductor regions. The method furtherincludes forming a hyperbolic metamaterial structure in the secondsemiconductor region such that the hyperbolic metamaterial structure iscoupled to the quantum well layer. Forming the hyperbolic metamaterialstructure in the second semiconductor region may include forming acavity in the second semiconductor region, the hyperbolic metamaterialstructure being formed in the cavity. In some embodiments, forming thehyperbolic metamaterial structure in the second semiconductor region mayinclude growing a first portion of the second semiconductor region,forming the hyperbolic metamaterial structure on the portion, andgrowing a second portion of the second semiconductor region over thehyperbolic metamaterial structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described in conjunction with thedrawings, in which:

FIG. 1A is a side cross-sectional view of an LED including a hyperbolicmetamaterial (HMM) structure in accordance with an embodiment of thepresent disclosure;

FIG. 1B is a top view of the LED of FIG. 1A;

FIG. 2A is a side cross-sectional view of an LED including an HMM array;

FIG. 2B is a top view of the LED of FIG. 2A;

FIG. 3 is a plot of spectral gain of the LED's p-n junction superimposedwith a plot of plasmonic resonance of the HMM array of FIGS. 2A and 2B;

FIG. 4 is a top view of an LED with a one-dimensional HMM array having aspatially variant pitch;

FIG. 5 is a top view of an LED with a one-dimensional HMM array having aspatially variant duty cycle;

FIG. 6A is a top view of an LED with a two-dimensional HMM array havinga spatially variant coupling of the HMM array features to the quantumwell layer;

FIG. 6B is a top view of the LED of FIG. 6A;

FIG. 6C is a plot of the spatially variant distance to the quantum welllayer and the corresponding spatially variant coupling of the HMM arrayfeatures to the quantum well layer of the LED of FIG. 6A;

FIG. 7 is a top view of an LED with a two-dimensional HMM array having aplurality of concentric features;

FIG. 8 is a top view of an LED with a two-dimensional HMM array having aplurality of sub-arrays of features, each sub-array including aone-dimensional array of grating lines;

FIG. 9 is a top view of an HMM array having a plurality of cross-shapedfeatures;

FIG. 10 is a top view of an HMM array having a plurality of intersectingchevron features;

FIG. 11A is a side cross-sectional view of an LED with HMM featuresinclined towards the array center;

FIG. 11B is a top view of the LED of FIG. 11A;

FIG. 12 is a side cross-sectional view of a display device including anarray of LEDs of FIGS. 11A and 11B;

FIGS. 13A to 13K are side cross-sectional views of an LED of the presentdisclosure at various progressing stages of manufacture, superimposedwith corresponding steps of a flow chart illustrating a correspondingmethod of manufacture of the LED;

FIGS. 14A to 14D are side cross-sectional views of an LED of the presentdisclosure at various progressing stages of manufacture in accordancewith an alternative embodiment, superimposed with steps of a flow chartillustrating a corresponding alternative method of manufacture of theLED; and

FIG. 15 is an isometric view of a head-mounted display of the presentdisclosure.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives and equivalents, as will be appreciatedby those of skill in the art. All statements herein reciting principles,aspects, and embodiments of this disclosure, as well as specificexamples thereof, are intended to encompass both structural andfunctional equivalents thereof. Additionally, it is intended that suchequivalents include both currently known equivalents as well asequivalents developed in the future, i.e., any elements developed thatperform the same function, regardless of structure.

As used herein, the terms “first”, “second”, and so forth are notintended to imply sequential ordering, but rather are intended todistinguish one element from another, unless explicitly stated.Similarly, sequential ordering of method steps does not imply asequential order of their execution, unless explicitly stated. In FIGS.1A-1B, 2A-2B, FIGS. 4, 5, 6A-6B, FIGS. 7-10, 11A-11B, FIG. 12, FIGS.13A-13K, and FIGS. 14A-14D, similar reference numerals denote similarelements.

In accordance with the present disclosure, a light emitting efficiencyof an LED device may be increased by shortening a lifetime of excitedenergy states of electron-hole pairs in the LED's p-n junction byenhancing or accelerating radiative recombination of the electron-holepairs. Such enhancement may be achieved by providing a hyperbolicmetamaterial (HMM) structure in the vicinity of a quantum well layer atthe p-n junction of the LED. For efficient energy transition from thequantum wells to the HMM structure, the latter may be configured to havea plasmonic resonance at an optical frequency within a spectral gainband of the quantum well layer. Furthermore, the HMM structure may beimpedance-matched to a surrounding medium to facilitate radiativerelaxation of surface plasmons and to increase the light output. The HMMstructure may be patterned to provide an arrayed structure facilitatingthe emission of light at a certain pre-defined polarization(s), and/orin a certain pre-defined direction(s).

Referring to FIGS. 1A and 1B, a light-emitting diode (LED) 100 includesa first semiconductor region 101 of n-type of conductivity and a secondsemiconductor region 102 of p-type of conductivity disposed on the firstsemiconductor region 101 and forming a p-n junction 104 with the firstsemiconductor region 101. In some embodiments, the LED 100 may be asilicon diode having n- and p-doped silicon semiconductor regions.Dopants for silicon LED 100 may include e.g. boron, gallium, aluminum,or indium (p-dopants), and arsenic, phosphorus, bismuth, antimony, orlithium (for n-dopants). In some embodiments, the LED 100 may be basedon gallium nitride (GaN). Dopants for GaN LED 100 may include e.g.magnesium (p-dopant), and silicon, germanium, or carbon (n-dopants).These examples are non-limiting, and many other material combinationsare possible.

A quantum well layer 106 is disposed at the p-n junction 104 between thefirst 101 and second 102 semiconductor regions. The quantum well layer106 may include a multi-layer stack including e.g. gallium arsenidealternating with a material with a wider bandgap, like aluminumarsenide. Such structures can be grown by molecular beam epitaxy orchemical vapor deposition with control of the layer thickness down tomonolayers, that is, a single-atom layers of materials. Thin metal filmsmay also support quantum well states in some cases. Bottom 110 and top112 electrodes may be provided in a suitable geometry. For example, theLED 100 has the bottom electrode 110 in form of a continuous layer, andthe top electrode 112 as a frame around the perimeter of the secondsemiconductor region 102. Numerous other configurations are possible.Lateral dimensions a and b of the LED 100 (FIG. 1B) are small forminiature diodes, e.g. no greater than 25×25 micrometers, and even nogreater than 10×10 micrometers. The bottom semiconductor region, i.e.the first semiconductor region 101 may be of p-type and the second ortop semiconductor region 102 may be of n-type. More generally, the firstsemiconductor region 101 is of one of p- or n-conductivity types, andthe second semiconductor region is of the other one of p- orn-conductivity types.

A hyperbolic metamaterial (HMM) structure 108 may be disposed in thesecond semiconductor region 102 and optically coupled to the quantumwell layer 106. The HMI structure 108 may include a stack of interposedthin metal and dielectric, or metal and semiconductor layers, or metal,dielectric, and semiconductor layers. The materials and thicknesses ofthe layers may be selected such as to engineer a desired dielectricpermittivity for emitted light 114 polarized in a plane p perpendicularto the metamaterial layers. In some embodiments, the hyperbolicmetamaterial structure 108 includes interposed layers of metal such asgold or silver, and dielectric such as metal or semiconductor oxide,semiconductor such as silicon and dielectric, metal and semiconductor,etc. The metamaterial layer may include a plurality of nanoantennasdisposed in a one-dimensional or two-dimensional array, and/or aplurality of nanowires e.g. less than 20 nm to 700 nm long with a 10 nmto 75 nm in lateral dimension, with a separation between individualnanowires of below 100 nm. The nanowires may have a circularcross-section or a non-circular cross-section, such as a square, arectangular, and/or an elliptical cross-section. Nanoantenna embodimentswill be discussed further below.

In operation, a voltage is applied between the bottom 110 and top 112electrodes with the higher electrical potential applied to the topelectrode 112 and the lower electrical potential applied to the bottomelectrode 110. This causes the p-n junction 104 to be forward biased,creating a force on the electrons pushing them from the firstsemiconductor region 101 toward the second semiconductor region 102.With forward bias, electrons enter the p-n junction 104 and recombinewith holes at the quantum well layer 108. The quantum well layer 106provides intermediate energy states where electrons and holes canradiatively recombine with each other. The hyperbolic metamaterialstructure 108 may act to remove the radiated energy from the quantumwells 106. Due to this process, the HMI structure 108 shortens thecarrier recombination lifetime and thereby increases the lightconversion efficiency of the LED 100.

Referring to FIGS. 2A and 2B, an LED 200 is an embodiment of the LED 100of FIGS. 1A and 1B. An arrayed HMI structure 208 of the LED 200 of FIGS.2A and 2B is similar to the HMI structure 108 of FIGS. 1A and 1B, inthat it includes a stack of alternating metal-dielectric ormetal-semiconductor layers. The arrayed HMI structure 208 of FIGS. 2Aand 2B includes a two-dimensional array 227 of features 228 disposed inthe second semiconductor region 102 and coupled to the quantum welllayer 106. The features 228 may have a variety of sizes and shapes, e.g.rectangular as shown, polygonal, rhombic, square, circular, oval, etc.,while retaining the basic multi-layer HMM structure. The desired patternof the features 228 may be achieved e.g. by a selective etching process.The size, shape, and composition of the features 228 enables one to tuneplasmonic resonance of the features 228, and therefore to further tuneoptical properties of the entire hyperbolic metamaterial structure 208to match a pre-defined set of optical properties related to complexdielectric permittivity, polarization properties, etc. For example, theshape, size, and composition of the features 228 may be selected toprovide a plasmonic resonance at wavelengths of color channels of avisual display, such as red, green, and blue channels. Furthermore, thespectral width and, to a certain degree, a spectral shape of theplasmonic resonance may be selected to match a pre-defined target value.

Referring to FIG. 3, a spectral gain curve 302 of the p-n junction 104of the LED 200 of FIGS. 2A and 2B is superimposed with a plasmonicresonance absorption curve 304 of the arrayed HMI structure 208. Forvisible light LEDs, a full width at half maximum (FWHM) of the gainbandwidth may be about 40 nm, or approximately 43 THz for a green colorchannel centered at 530 nm. The plasmonic resonance FWHM depends on thematerials used in the HMI stack. For example, when silver is used as themetal in HMM stack, the plasmonic resonance FWHM may be as narrow as 15nm, or approximately 16 THz for the green color channel at 530 nm. Thismay allow the light energy to be channeled into a narrower emissionbandwidth due to the effect of quicker radiative depletion of the 15 nmband of the spectral gain profile of an LED's p-n junction. That enablesthe spectral bandwidth of an LED display's color channel to be madenarrower by using HMM structures. Narrower color channel bandwidths mayresult in brighter, more saturated displayed colors. Narrower colorchannel bandwidths may also simplify the task of focusing andredirecting the display light using wavelength-selective redirectingelements, such as diffraction gratings, for example, and may enable theuse of diffractive optical elements having focusing or defocusing i.e.optical power. Furthermore, the arrayed HMM structure 208 may enabletuning of the emitted spectral bandwidth and position of maximumemission wavelength. In some embodiments, the plasmonic resonance tuningof the arrayed HMI structure 208 may also allow tuning of lightextraction efficiency, which creates some interesting possibilities,such as controllable near-field apodization of a light beam emitted bythe LED 200.

Turning to FIG. 4, an LED 400 is similar to the LED 200 of FIGS. 2A and2B in that it includes an HMM structure coupled to a quantum wells layerat a p-n junction, not shown. The HMM structure of the LED 400 includesa one-dimensional array 427 of HMM lines 428, i.e. a one-dimensionalgrating with the grating lines made of HMM stack of layers, e.g. a stackof alternating thin metal and dielectric layers, or a stack ofalternating thin semiconductor and dielectric layers, or a stack ofalternating thin metal and semiconductor layers, etc. Interline gaps 429may be filled with the semiconductor of the second type e.g. p-type inthis example. The materials and thicknesses of the layers comprising theHMM lines 428 may be selected so as to provide a desired complexdielectric permittivity of the HMM lines 428. The dielectricpermittivity of the HMM lines 428 is generally polarization dependent.The grating may be a subwavelength grating having a pitch p less thanwavelength of light, or a multi-order diffraction grating with the pitchp greater than the wavelength. Herein, the pitch p is defined as adistance 411 between centers of the neighboring HMM lines 428, asillustrated in FIG. 4. The optical performance of the one-dimensionalarray 427 of the HMM lines 428 depends not only on the complexdielectric permittivity of the HMM stack but also on the pitch p and aduty cycle d defined as d=t/p, where t is the line width, as shown inFIG. 4. The one-dimensional array 427 of the HMM lines 428 has avariable pitch p in going along x-axis. For subwavelength gratings, thevariable pitch p may provide an optical phase of the emitted radiationvariable along x-axis, which may enable configuring a far-field opticalpower density distribution of the LED 400. For over-wavelength gratings,i.e. diffraction gratings capable of diffracting light into a pluralityof diffraction orders, the variable pitch p may provide the control offar-field light distribution of the LED 400 via non-zero diffractionorders.

Referring to FIG. 5, an LED 500 is similar to the LED 400 of FIG. 4 inthat it includes a patterned HMM structure coupled to quantum wellslayer at a p-n junction, not shown. The HMM structure of the LED 500includes a one-dimensional array 527 of HMM grating lines 528, i.e. aone-dimensional grating with the grating lines comprising an HMM stackof layers, e.g. a stack of alternating thin metal and dielectric layers,or a stack of alternating thin semiconductor and dielectric layers, or astack of alternating thin metal and semiconductor layers, etc. Interlinegaps 529 may be filled with the semiconductor of the second type e.g.p-type. The materials and thicknesses of the layers of the HMM gratinglines 428 may be selected so as to provide a desired complex dielectricpermittivity of the HMM grating lines 428. The one-dimensional array 527may be a subwavelength or over-wavelength grating. It is seen that theone-dimensional array 527 of the HMM grating lines 528 has a constantpitch p but variable duty cycle d defined as d=t/p, in going alongx-axis in FIG. 5. Herein, the pitch p is defined as a distance 511between centers of the neighboring HMM lines 528, as illustrated in FIG.5. For subwavelength gratings, the variable duty cycle d may providevariable optical amplitude and phase of the emitted radiation, which mayenable configuring a far-field optical power density distribution of theLED 500. For over-wavelength gratings, i.e. diffraction gratings capableof diffracting light into a plurality of diffraction orders, thevariable pitch p may provide the control of far-field light distributionof the LED 500 via non-zero diffraction orders.

Referring now to FIGS. 6A and 6B, an LED 600 is an embodiment of the LED200 of FIGS. 2A and 2B. An HMM structure 608 of the LED 600 of FIGS. 6Aand 6B includes a two-dimensional array 627 of HMM features 628 disposedin the second semiconductor region 102 and coupled to the quantum welllayer 106. A vertical gap d between the quantum well layer 106 and theHMM features 628 of the two-dimensional array 627 varies across thetwo-dimensional array 627, i.e. is different in areas 630, 632, and 634(FIG. 6B) of the two-dimensional array 627. In the embodiment shown, thevertical gap d is largest in the outmost area 630, is smallest at thecenter area 634, and is intermediate in the intermediate area 632. Theintermediate area 632 and the center area 634 are denoted in FIG. 6Bwith thick dashed lines. The different levels of the HMM features 628may be obtained by first selective etching the second semiconductorregion 102 to different depth in the areas 630, 632, and 634, anddepositing the HMM into the resulting multi-level structure.

FIG. 6C shows a lateral variation 640 of the vertical gap d across theLED 600. The lateral variation 640 is illustrated in FIG. 6C with dashedlines. The variable vertical gap d causes the optical coupling betweenthe features 628 of the two-dimensional array 627 and the quantum welllayer 106 to vary across the two-dimensional array 627. Smaller verticalgap d results in a stronger optical coupling, and a larger vertical gapd results in a weaker optical coupling. This is illustrated by acoupling curve 642 (solid line). Stronger coupling results in a strongeremitted light field. As a result, the light beam emitted by the LED 600is apodized. The apodization of the light beam may suppress slidelobesof the optical far field profile, thereby making the emitted opticalbeam more uniform in angular power density distribution.

Dielectric properties of an HMM layer are typically polarizationdependent. For example, an HMM layer may behave like a metal for opticalpolarization directed along the HMM surface, and like a dielectric or asemiconductor for optical polarization directed across the HMM surface.On the other hand, plasmonic resonance characteristics for smallsub-wavelength features or particles may be dependent on the featureshape. Therefore, by configuring the sub-wavelength HMM features of anHMM array of features, that is, by providing a certain shape and/ororientation of the HMM features in the array of features, desiredpolarization properties of the emitted light may be achieved. By way ofa non-limiting example, referring to FIG. 7, an LED 700 includes atwo-dimensional HMM array 727 having a plurality of concentric features728. Each concentric feature 728 may include a plurality of concentricHMM rings, and a center feature 728 may be an HMM cylinder. Theconcentric HMM rings may be separated by a dielectric or may be filledwith the semiconductor of the second type e.g. p-type. Since theconcentric features 728 are rotationally symmetric about an axisperpendicular to the emitting plane of the LED 700, the polarization ofthe emitted light may be circular, or may be unpolarized, depending onthe geometry of the concentric features 728 of the two-dimensional HMMarray 727.

Referring to FIG. 8, an LED 800 includes a two-dimensional HMM array 827including a plurality of sub-arrays 825 of features. Each sub-array 825may include its own structure e.g. a linear array of grating lines 828,which may be inclined as shown. The grating lines 828 may be disposed ina dielectric or in a semiconductor region of the LED 800. The HMM array827 may be used to provide a linear polarization of emitted light. Moregenerally, features of an HMM array may be configured to provide adesired pre-defined polarization of emitted light.

It is noted that the HMM feature shapes described above are onlynon-limiting illustrative examples of the shapes that may be used. Othershapes may be used as well; for example, referring to FIG. 9, an HMMarray 927 is a two-dimensional array of cross-shaped features. FIG. 10illustrates am HMM array 1027 including arrays of chevron shapes, whichmay cross each other as shown, forming an array of double-chevronfeatures. The size of the features of HMM arrays described above may besub-microscopic. Each feature of the array is a nanoantenna having aplasmonic resonance in the optical part of electromagnetic spectrum.Plasmonic resonance of specific shapes may be computed using a suitableelectromagnetic simulation software. Nanoantennas may be constructed tohave a plasmonic resonance corresponding to an optical frequency of ared, green, or blue color channel in the visible part of the opticalspectrum.

Referring to FIGS. 11A and 11B, an LED 1100 is an embodiment of the LED200 of FIGS. 2A and 2B and the LED 600 of FIGS. 6A and 6B. An HMMstructure 1108 of the LED 1100 of FIGS. 11A and 11B includes atwo-dimensional array 1127 of HMM features 1128 disposed in the secondsemiconductor region 102 and optically coupled to the quantum well layer106. In some embodiments, the HMM features 1128 may extend from thequantum well layer 106. At least some of the HMM features 1128 areinclined towards a centerline 1150 of the two-dimensional array 1127(FIG. 11A), i.e. towards a central feature 1128 a of the two-dimensionalarray 1127 (FIG. 11B). In some embodiments, HAM features 1128 areinclined in a same direction, e.g. towards one side of the LED 1100.

In operation, each feature 1128 functions as a nanoantenna creating anoptical near field propagating along the feature 1128. This causes theemitted light to focus at some distance above the LED 1100, forming abeam waist w above the LED 1100. This is an example of how shaping ofindividual features 1128 may enable output beam shaping, which issometimes referred to as “chief ray engineering”. The shapes ofindividual nanoantennas may be rectangular, rhomboidal, polygonal,circular, oval, chevron-like, etc. The nanoantennas, i.e. the features1128, may be inclined not towards the center but all point to a certainpre-defined direction.

Turning to FIG. 12, a display device 1200 may use an array of LEDs1220-1 . . . 1220-n, similar to the LED 1100 of FIGS. 11A and 11B. Thearray may be one- or two-dimensional. The LEDs 1220-1 . . . 1220-n maybe disposed on a common substrate 1262 which can be, for example, acommon n-type substrate for semiconductor junctions of the LEDs 1220-1 .. . 1220-n. The LEDs 1220-1 . . . 1220-n include respective HAIM arrays1227-1 . . . 1227-n optically coupled to respective quantum well layers,not shown. HMI arrays 1227-1 . . . 1227-n of the LEDs 1220-1 . . .1220-n may be configured to form optical beams 1229-1 . . . 1229-nconverging to a common point. An electrode structure and/or acontrolling gate structure in a multiplexed drive configuration may beprovided for independently energizing each LED 1220-1 . . . 1220-n ofthe LED array. The electrode/gate structure is not shown in FIG. 12 forsimplicity.

The display device 1200 further includes a collimator element havingoptical i.e. focusing power, e.g. a lens 1260, optically coupled to theLEDs 1220-1 . . . 1220-n at the common converging point of the opticalbeams 1229-1 . . . 1229-n. The lens 1260 is spaced apart from the LEDs1220-1 . . . 1220-n, e.g. by one focal length f of the lens 1260, and isconfigured for collimating and redirecting the optical beams 1129-1 . .. 1229-n. Since the optical beams 1129-1 . . . 1229-n are converging toa common point, the size and weight of the lens 1260 may be considerablyreduced. The individual optical beams 1129-1 . . . 1229-n may also berefocused or reshaped for a better matching to the clear aperture of thelens 1260. This is an example of how chief ray engineering enables oneto increase throughput while saving space, weight, and manufacturingcosts of the display device 1200.

Referring now to FIGS. 13A to 13K, a manufacturing process of an LED ofthe present disclosure includes providing (1352; FIG. 13A) a substrate1300, for example silicon or gallium nitride (GaN) substrate. A firstsemiconductor region 1301 is formed, e.g. epitaxially grown (1354; FIG.13B), on the substrate 1300. The first semiconductor region 1301 may beof one of p- or n-conductivity types. A quantum well layer 1306 may thenbe epitaxially grown (1356; FIG. 13C) on the first semiconductor region1301. The quantum well layer 1306 may include a multi-layer stack ofgallium arsenide alternating with a material with a wider bandgap, likealuminum arsenide. The quantum well layers may be grown by molecularbeam epitaxy or chemical vapor deposition with control of the layerthickness down to monolayers, that is, a single-atom layers ofmaterials. Thin metal films may also support quantum well states in somecases. These are non-limiting examples of material systems that may beused.

A second semiconductor region 1302 is formed, e.g. grown (1358; FIG.13D), on the quantum well layer 1306. The second semiconductor region1302 may be of the other one of p- or n-conductivity types. In otherwords, if the first semiconductor region 1301 is of n-type, then thesecond semiconductor region 1302 is of p-type, and vice versa. In someembodiments, the LED 100 may be a silicon diode having n- and p-dopedsilicon semiconductor regions. Dopants for silicon LED 100 may includee.g. boron, gallium, aluminum, or indium (p-dopants), and arsenic,phosphorus, bismuth, antimony, or lithium (for n-dopants). The LED 100may be also based e.g. on gallium nitride (GaN). Dopants for GaN LED 100may include e.g. magnesium (p-dopant), and silicon, germanium, or carbon(n-dopants). These examples are non-limiting, and many other materialcombinations may be used.

The manufacturing process may further include forming (1360; FIG. 13E) acavity 1380 in the second semiconductor region 1302. The cavity 1380 maybe formed by a suitable etching process, e.g. reactive ion etching(RIE). A hyperbolic metamaterial (HMM) structure 1308 is then formede.g. epitaxially grown (1362; FIG. 13F) in the cavity 1380. The HMMstructure 1308 may then be patterned (1364; FIG. 13G) in the cavity 1380to provide a one- or two-dimensional array of HMM nanoantennas describedabove. The patterning may be performed e.g. by a masked etching process.The second semiconductor region 1302 may then be grown back (1366; FIG.13H) to fill the cavity 1380.

The obtained structure may then be passivated (1368; FIG. 13I) e.g. bygrowing a top oxide layer 1303. The passivated LED structure may beetched (1370; FIG. 13J) to crease a mesa for subsequent electrodeapplication. Electrical contacts 1305 may then be formed (1372; FIG.13K) through vias in the top oxide layer 1303. The electrode structureincluding the electrical contacts 1305 is only shown as an illustrativeexample.

Referring to FIGS. 14A to 14D, an alternative embodiment of forming theHMM structure 1308 is presented. The steps illustrated in FIGS. 14A to14D replace steps 1358 to 1366 shown in FIGS. 13D to 13H. The method mayinclude growing (1402; FIG. 14A) a first portion of the secondsemiconductor region 1302 to a small thickness, e.g. 1 nm to 10 nmthick, and then growing (1404; FIG. 14B) the HMM structure 1308 on thethin second semiconductor region 1302. The HMM structure 1308 may the bepatterned (1406; FIG. 14C) using a suitable etching technique to providethe HMM structure 1308, which may include, for example, a one- ortwo-dimensional array of HMM nanoantennas described above. A secondportion of the second semiconductor region 1302 may then be grown, tothe full thickness of the second semiconductor region 1302 (1408; FIG.14D) over the HMM structure 1308, as shown. It is further noted that themanufacturing methods presented above are only illustrative examples.HMM LED structures disclosed herein may be manufactured using a broadvariety of methods.

Embodiments of the present disclosure may include, or be implemented inconjunction with, as light sources for an artificial reality system. Anartificial reality system adjusts sensory information about outsideworld obtained through the senses such as visual information, audio,touch (somatosensation) information, acceleration, balance, etc., insome manner before presentation to a user. By way of non-limitingexamples, artificial reality may include virtual reality (VR), augmentedreality (AR), mixed reality (MR), hybrid reality, or some combinationand/or derivatives thereof. Artificial reality content may includeentirely generated content or generated content combined with captured(e.g., real-world) content. The artificial reality content may includevideo, audio, somatic or haptic feedback, or some combination thereof.Any of this content may be presented in a single channel or in multiplechannels, such as in a stereo video that produces a three-dimensionaleffect to the viewer. Furthermore, in some embodiments, artificialreality may also be associated with applications, products, accessories,services, or some combination thereof, that are used to, for example,create content in artificial reality and/or are otherwise used in (e.g.,perform activities in) artificial reality. The artificial reality systemthat provides the artificial reality content may be implemented onvarious platforms, including a wearable display such as an HMD connectedto a host computer system, a standalone HMD, a near-eye display having aform factor of eyeglasses, a mobile device or computing system, or anyother hardware platform capable of providing artificial reality contentto one or more viewers.

Referring to FIG. 15, an HMD 1500 is an example of an AR/VR wearabledisplay system. The HMD 1500 can include any of the LEDs or displaydevices described herein. The LEDs may be used as light sources forilluminating the user's face, as a light source for a visual display, asdisplay pixels, etc. The function of the HMD 1500 is to augment views ofa physical, real-world environment with computer-generated imagery,and/or to generate the entirely virtual 3D imagery. The HMD 1500 mayinclude a front body 1502 and a band 1504. The front body 1502 isconfigured for placement in front of eyes of a user in a reliable andcomfortable manner, and the band 1504 may be stretched to secure thefront body 1502 on the user's head. A display system 1580 may bedisposed in the front body 1502 for presenting AR/VR imagery to theuser. Sides 1506 of the front body 1502 may be opaque or transparent.

In some embodiments, the front body 1502 includes locators 1508 and aninertial measurement unit (IMU) 1510 for tracking acceleration of theHMD 1500, and position sensors 1512 for tracking position of the HMD1500. The IMU 1510 is an electronic device that generates dataindicating a position of the HMD 1500 based on measurement signalsreceived from one or more of position sensors 1512, which generate oneor more measurement signals in response to motion of the HMD 1500.Examples of position sensors 1512 include: one or more accelerometers,one or more gyroscopes, one or more magnetometers, another suitable typeof sensor that detects motion, a type of sensor used for errorcorrection of the IMU 1510, or some combination thereof. The positionsensors 1512 may be located external to the IMU 1510, internal to theIMU 1510, or some combination thereof.

The locators 1508 are traced by an external imaging device of a virtualreality system, such that the virtual reality system can track thelocation and orientation of the entire HMD 1500. Information generatedby the IMU 1510 and the position sensors 1512 may be compared with theposition and orientation obtained by tracking the locators 1508, forimproved tracking accuracy of position and orientation of the HMD 1500.Accurate position and orientation is important for presentingappropriate virtual scenery to the user as the latter moves and turns in3D space.

The HMD 1500 may further include a depth camera assembly (DCA) 1511,which captures data describing depth information of a local areasurrounding some or all of the HMD 1500. To that end, the DCA 1511 mayinclude a laser radar (LIDAR), or a similar device. The depthinformation may be compared with the information from the IMU 1510, forbetter accuracy of determination of position and orientation of the HMD1500 in 3D space.

The HMD 1500 may further include an eye tracking system 1514 fordetermining orientation and position of user's eyes in real time. Theobtained position and orientation of the eyes also allows the HMD 1500to determine the gaze direction of the user and to adjust the imagegenerated by the display system 1580 accordingly. In one embodiment, thevergence, that is, the convergence angle of the user's eyes gaze, isdetermined. The determined gaze direction and vergence angle may also beused for real-time compensation of visual artifacts dependent on theangle of view and eye position. Furthermore, the determined vergence andgaze angles may be used for interaction with the user, highlightingobjects, bringing objects to the foreground, creating additional objectsor pointers, etc. An audio system may also be provided including e.g. aset of small speakers built into the front body 1502.

The hardware used to implement the various illustrative logics, logicalblocks, modules, and circuits described in connection with the aspectsdisclosed herein may be implemented or performed with a general purposeprocessor, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA) orother programmable logic device, discrete gate or transistor logic,discrete hardware components, or any combination thereof designed toperform the functions described herein. A general-purpose processor maybe a microprocessor, but, in the alternative, the processor may be anyconventional processor, controller, microcontroller, or state machine. Aprocessor may also be implemented as a combination of computing devices,e.g., a combination of a DSP and a microprocessor, a plurality ofmicroprocessors, one or more microprocessors in conjunction with a DSPcore, or any other such configuration. Alternatively, some steps ormethods may be performed by circuitry that is specific to a givenfunction.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments andmodifications, in addition to those described herein, will be apparentto those of ordinary skill in the art from the foregoing description andaccompanying drawings. Thus, such other embodiments and modificationsare intended to fall within the scope of the present disclosure.Further, although the present disclosure has been described herein inthe context of a particular implementation in a particular environmentfor a particular purpose, those of ordinary skill in the art willrecognize that its usefulness is not limited thereto and that thepresent disclosure may be beneficially implemented in any number ofenvironments for any number of purposes. Accordingly, the claims setforth below should be construed in view of the full breadth and spiritof the present disclosure as described herein.

What is claimed is:
 1. A light-emitting diode (LED) comprising: a firstsemiconductor region of one of p- or n-conductivity types; a secondsemiconductor region of the other one of p- or n-conductivity types,forming a p-n junction with the first semiconductor region; a quantumwell layer at the p-n junction between the first and secondsemiconductor regions; and a hyperbolic metamaterial structure in thesecond semiconductor region; wherein the hyperbolic metamaterialstructure comprises an array of features coupled to the quantum welllayer, the array of features having a spatially variant pitch.
 2. TheLED of claim 1, wherein the hyperbolic metamaterial structure comprisesa stack of alternating metal and semiconductor layers.
 3. The LED ofclaim 1, wherein the LED has lateral dimensions of no greater than 25×25micrometers.
 4. The LED of claim 1, wherein the spatially variant pitchis less than a wavelength of light emitted by the LED.
 5. The LED ofclaim 1, wherein the array of features has a plasmonic resonance opticalfrequency within a spectral gain band of the quantum well layer.
 6. TheLED of claim 1, wherein the array of features has a spatially varyingduty cycle.
 7. The LED of claim 1, wherein a gap between the quantumwell layer and features of the array varies across the array.
 8. The LEDof claim 1, wherein the array of features is two-dimensional.
 9. The LEDof claim 1, wherein each feature of the array comprises at least one ofa cylinder feature, a cross feature, or a chevron feature.
 10. The LEDof claim 1, wherein the array comprises a plurality of sub-arrays offeatures.
 11. The LED of claim 10, wherein each sub-array comprises anarray of grating lines.
 12. The LED of claim 1, wherein the features ofthe array are configured to provide a pre-defined polarization ofemitted light.
 13. The LED of claim 1, wherein features of the arrayextend from the quantum well layer and into the second semiconductorregion.
 14. The LED of claim 1, wherein at least some of the features ofthe array are inclined towards a center of the array.